1. Field of Invention
The present invention relates to a color image processing apparatus for processing an image signal obtained by reading a color image or a transmitted image signal and outputting the processed image signal to a printer or a display and, more particularly, to a color image processing apparatus having a function of correcting a color image signal obtained by reading a color image signal photographed by a camera using a silver halide film or a color image signal read by a reading apparatus, a brightness reference of which is unknown, by estimating the brightness of an original image.
2. Description of Related Art
In general, when an original color image is to be reproduced by an output apparatus such as a color printer or a color display from a color image signal obtained by reading a color image, photographed by a camera using a negative or positive color film, by using an image sensor or a color image signal obtained by photographing performed by an electronic camera, the reproducibility of the color image is significantly reduced if the brightness of a photographing state is inadequate. This is mainly because the dynamic range in gradation of an output apparatus is narrower than those of a photographing system and a reading system. For example, if an original image is dark, a dark color portion may not be reproduced because it becomes continuous. On the other hand, if an original image is too bright, a light color portion may not be reproduced because it is whitened.
Similarly, when an original color image is to be reproduced from a color image signal read by a reading apparatus having an unknown brightness reference or a transmitted color image signal, the reproducibility is reduced unless the white reference level of the reading apparatus coincides with that of an output apparatus.
To avoid the above inconvenience, the brightness of an original color image is estimated from an input color image signal, and the color image signal is corrected on the basis of the estimated brightness. As this correction method, a method called an averaging method or an equivalent neutral density method is known. This method is based on the statistical fact that the total sum of the color densities of an entire color image to be reproduced is close to neutral gray. In this method, therefore, an average value of the color densities of an entire image is calculated, and a brightness is estimated from this average value, thereby executing correction. In this method, however, if an original image is in a color deviated from gray, it is difficult to correctly estimate the brightness. In addition, since the total sum of the color densities of an entire image is calculated, a brightness cannot be estimated unless a color image signal of an entire image is input. Therefore, to correct color image signals transmitted at a predetermined rate, a large-capacity memory is required to temporarily store color image signals of an entire image, and response characteristics are degraded.
Published Unexamined Japanese Patent Application No. 63-131777 describes a method in which a maximum value of one (e.g., an R signal) of color image signals (generally, three primary color signals R, G, and B) or a signal corresponding to a luminance is calculated and compared with a predetermined reference value to obtain a difference, thereby generating brightness information on the basis of this difference. Also in this method, however, if a large amount of colors (e.g., a scene of lawn constituted by green) deviated from gray is included in an original color image, it is difficult to correctly estimate the brightness.